Chapter 2. Introduction to the IS-IS Routing Protocol

To understand the innards and underlying operational principles of the Intermediate System-to-Intermediate System (IS-IS) routing protocol, it is important to place it in perspective within the overall scheme of internetworking protocols and related technologies. This chapter provides insight into the roots of the IS-IS protocol and discusses the connectionless networking environment that it was envisioned to support by the International Organization for Standardization (ISO).

The chapter begins with an introduction to the Open System Interconnection (OSI) reference model and then briefly discusses the two types of data communications services specified by ISO within the framework of the OSI reference model: Connection Network Service ( CONS ) and Connectionless Network Service ( CLNS ). IS-IS was designed as part of the CLNS environment to provide the necessary intelligence for automatic and dynamic routing of data packets in ISO CLNS networking environments. From its inception, the IS-IS protocol has been adapted for IP routing and other capabilities, such as traffic engineering with Multiprotocol Label Switching (MPLS).